Technical field

It is an object of the present invention to provide a method for detecting ions in an ion trap and further to provide an ion trap for trapping and detecting ions.

Background

Ion trap mass spectrometry sees usage semiconductor fabrication techniques and related fields in order to monitor plasma material processing. Therein, an ion trap is attached to a vacuum apparatus collecting ions and mass selectively detecting ions in the trapping volume. The detected ions can be identified by their mass with indication of species of gases being part of the fabrication process for example and as such being present in the vacuum apparatus. Process information, including product failures, apparatus failures or the like can be detected and monitored.

A Radio Frequency (RF) Ion Storage Mass spectrometer can be designed to operate using a variety of different technologies and detection algorithms. Most early tools worked by directing ions to charged particle detectors such as electron multipliers for destructive detection. Transfer of the ions to the detector could happen via a variety of different techniques such as scanning instability or resonant excitation via the application of extra monopole, dipole, or quadrupole excitations. Later tools used very sensitive electronics for image current detection of induced ion motion.

What these techniques have in common is the loss of the ability to dynamically measure ion response to external stimulus. Destructive particle detector techniques must supply enough energy to the ions to eject them from the trap volume, but do not measure ion signals prior to their collision with the detector. FFT image current techniques typically use the same electronics for stimulation and detection. However, stimulation and detection must be time multiplexed because unavoidable crosstalk from the stimulation signal necessarily applied at the same frequency as that of stored ions would saturate the sensitive electronics used for detection.

Summary

It is an object of the present invention to overcome the drawbacks of the prior art and provide a detection scheme that allows for the non-destructive simultaneous stimulation and detection of ions in an ion trap.

The technical problem is solved by a method for detecting ions in an ion trap according to claim 1 and is further specified in the appendant dependent claims. Further, a solution to the given technical problems provided by an ion trap for trapping and detecting ions according to claim 14 and is further specified by the appendant dependent claims.

In a first aspect a method for detecting ions in an ion trap is provided. The method comprises the steps of: a) Ionizing ions in the ion trap or transferring them into the trap from an external ion source; b) Creating an RF storage field by applying a first RF signal to a first electrode of the ion trap; c) Applying an excitation signal to the ions in the ion trap; d) Detecting an image current signal induced to a second electrode, a third electrode or differentially between the second and third electrodes by oscillation of the ions excited by the excitation signal; wherein the excitation signal is a parametric excitation.

Therein, in accordance to the present invention, the excitation signal is a parametrical excitation. Thus, preferably the excitation signal has a different frequency than the oscillation frequency of the respective ions in the ion trap. In step a), ions in the ion trap are ionized in order to be responsive to the applied electric fields. Therein, ionization is a common technique and not part of the present invention. Any scheme applicable and matching to the demands of the present method can be used for ionizing. Example ionization techniques include electron impact ionization, photoionization, chemical ionization or the like.

In step b), a RF storage field is created in order to store the ionized ions. Preferably, the RF storage field is created by a first RF signal supply. The storage field is created by applying a first RF signal to a first electrode of the ion trap. Therein, step a) and step b) can be performed in the present order, in a reversed order (step b) before step a)), or simultaneously.

In step c), an excitation signal is applied to the ions in the ion trap. The excitation signal might be provided by a second RF signal supply or might be provided by a modulation of the first RF signal providing the RF storage field. Therein, the excitation signal might also be applied to the first electrode of the ion trap. Therein, according to the present invention, the excitation signal is a parametric excitation. A conventional monopolar or dipolar stimulus from the endcap electrodes may also applied to the ions prior to detection and prior to or concurrent with the parametric excitation signal so as to seed coherent motion for parametric amplification.

In step d) an image current signal is detected preferably by a detector wherein the image current signal is induced to a second electrode, a third electrode or alternatively being detected differently between the second and third electrodes, wherein the image current signal is caused by oscillation of the ions excited by the excitation signal.

Therein, excitation of the ions by the excitation signal is mass selective and only those ions are excited by the respective excitation signal having a specific and matching mass to charge ratio.

Thus, by the present invention ion excitation is facilitated by parametric excitation. Therein, the effect of parametric oscillation is used in order to separate the frequency of the excitation signal and the frequency of the image current signal induced into the second and/or third electrode. Thus, no time multiplexing of the excitation and detecting is necessary anymore due to the different frequencies. Thereby, the flexibility of the detecting scheme is enhanced and improved signal recovery options such as synchronous demodulation are now possible. Synchronous demodulation or lock-in detection, relies on comparison of the detected signal against a reference excitation signal or modulated excitation signal. Possible implementations include using a low frequency modulation (chopping) of excitation signal, direct multiplication of the ion signal against a calculated I-Q reference signal at a frequency that is half of the parametric excitation frequency, or for detailed tracking of specific peaks, various FM modulation techniques described in later sections.

Preferably, the ion oscillation frequency ω ₀ , relating to the secular frequency of the ions in the RF storage field and being dependent on the respective masses of the ions, is half the excitation frequency ω _excite i.e. ω _excite = 2ω ₀ .

Preferably, applying the excitation signal and detecting the image current signal are performed simultaneously. Time multiplexing of excitation and detecting is not necessary anymore and thus simultaneous detection can be performed without unintended interaction.

Preferably, application of the excitation signal is repeated at different frequencies sequentially. Thus, by applying the excitation signal with frequencies, different species of ions can be addressed, excited and subsequently detected. Alternatively, more than one excitation signal is applied having different frequencies. Therein, the excitation signals are superimposed and applied simultaneously in order to address at the same time different species of ions in order to detect them simultaneously.

Preferably, after applying a first excitation signal before applying a second excitation signal with a different frequency, ions excited by the first excitation signal are expelled from the ion trap. Thus, the already detected ions can be removed from the trapping volume of the ion trap for example by increasing the amplitude of the excitation signal beyond a certain point of stability causing the respective ions either collide with the surfaces of the ion trap or being forced out of the trapping volume. Thereby, the remaining species of ions in the trapping volume can be more reliably detected and superposition of response signals of different species is reduced. Preferably, the frequency of the excitation signal is swept over a predetermined range. Sweeping can be performed continuously or in discrete steps by shifting the frequency of the excitation signal or by changing the amplitude of the RF trapping field against a fixed parametric excitation frequency. Alternatively, sweeping can be performed stepwise or in a predetermined sequence in order to address specific ion species of interest or in a dynamically programmed way based on algorithmic tracking of peaks and pre-programmed logical rules. Thus, by sweeping a range of ions having different masses can be detected. Further, by sweeping the excitation signal, being now independent from the response signal of the excited ions, the response spectrum of the ions can be sampled over the frequency range.

Preferably, as a means of automatic gain control signal levels from selected peaks may be tuned to pre-chosen or dynamically calculated levels by real time adjustment of the excitation levels applied at their related frequencies and overall RF trapping voltage. This may be used, for example, to dynamically increase sensitivity during the course of single measurement.

Preferably, by sweeping the excitation signal over the ion response signal of the excited ions, sampling of the first derivative of the response signal of the excited ions can be determined. From the derivative signal the shape of the response signal of the ions can be calculated in order to determine the shape of the response signal being able to identify overlapping response signals from different ion species so-called isobaric species.

Preferably, when tracking a single m/z range, the frequency of the excitation signal frequency is modulated at a low frequency between an off-resonance frequency and an on-resonance frequency, wherein on-resonance frequency corresponds to two-times a frequency (due to parametric excitation) of the ion response signal. If ω _ion indicates a frequency corresponding to an excitation of a specific ion or ion species, on-resonance excitation frequency yields to 2 * ω _ion . The composite waveform formed from average response during each portion of the modulation cycle, i.e. the on-resonance frequency and the off- resonance frequency, is proportional to the peak height and may be synchronously demodulated against the original low frequency modulation pattern for further noise reduction.

Preferably, when detailed studies of a limited m/z range are desired the frequency of the excitation signal can be frequency modulated between two closely spaced frequencies and synchronously demodulated, with the center position being swept across the full mass range of interest during a single measurement. The demodulated response at any center frequency then corresponds to the first derivative of the m/z signal at that point, allowing for overlapping response signals from different species so-called isobaric species to be distinguished.

Preferably, the excitation signal ω _excite is between a low frequency ω _low and two-times a maximum frequency ω _max , i.e. ω _low ≤ ω _excite ≤ 2ω _max , wherein for the image current signal or ion response it yields ω _low /2 ≤ ω _ion ≤ ω _max , wherein “max being the upper frequency limit of the detector being the highest frequency the detector can deal with and ω _low being the cut-off frequency of the detector, i.e. the upper cut-off frequency of the detector. Thus, ω _low denotes the lowest frequency not being amplified by the detector. Further, ω _ion relates to the image current signal or response signal of the ion. Thus, image current signals in the range between ω _low /2 and ω _max can be determined dependent on the amplification range of the used detector. Alternatively, the image current signal ω _ion acquired upon detection is filtered wherein the signal at frequencies of the excitation signal are filtered out. Thus, a notch filter or band filter can be used dedicated to the frequency of the excitation signal. Since the excitation signal and the ion response signal are not at the same frequency due to parametric excitation, filtering of the excitation signal has no influence on the measured signal and thus influences of the excitation signal on the detection is avoided. Preferably, the excitation is modulated with a modulation frequency ^modulation smaller than ω ₀ , wherein the detection image current signal is demodulated with the modulation frequency. Thus, by modulation of the excitation signal, recovery schemes can be implemented for example in a phase-locked manner from the excitation signal. Therein, modulation can be applied to the amplitude of the excitation signal and/or the phase of the excitation signal.

Preferably, a phase sensitive detection of the image current signal is applied in order to reliably detect the response signal of the respective ions.

As a further aspect, an ion trap for trapping and detecting ions is provided, wherein the ion trap comprises a first electrode, a second electrode and a third electrode defining a trapping volume. Further, a first RF signal supply is connected to the first electrode and configured to generate an RF storage field. In addition, a second RF signal supply is connected to the first electrode and configured to generate an excitation signal. Therein, the second RF signal supply might be implemented as an additional module being additional to the first RF signal supply or might be integrated to the first RF signal supply thereby modulating the first RF signal provided by the first RF signal supply to apply the excitation signal to the ion trap. Preferably, the first RF signal supply and/or the second RF signal supply are implemented by one or more DDS modules (direct digital synthesis module). Therein, the excitation is a parametric excitation resulting in different frequencies of the excitation and the ion response signal induced in one or more of the electrodes. Further, the ion trap comprises a detector connected to the second electrode and/or the third electrode and configured to detect an image current signal induced by oscillation of the ions excited by the excitation signal. Therein, the detector might detect the image current induced to the second electrode or the third electrode or might detect the differential signal between the second electrode and third electrode.

Preferably, the first electrode is a ring electrode and the second electrode and third electrode are cap electrodes. Preferably, the ion trap is further built along the features described in connection with the method above.

Preferably, the method is further built along the features described in connection with the ion trap described above.

Thus, by the present ion trap and method, excitation of the ions in the ion trap by parametric excitation yields the advantage of separation of the excitation frequency and the ion response of the excited ions detected by the induced image current signal. Thus, no influences of the excitation signal on the detected image current signal appears. At the same time, simultaneous excitation and detection becomes feasible enabling different detections schemes and enhancing the sensitivity of the detection. In addition, it is possible to perform continuous application of the excitation signal leading to the possibility to measure the ion response over longer duration, leading to higher signal to noise levels and better mass resolution.

Figures

In the following the present invention is described in more detail with reference to the accompanied figures.

The figures show:

Figure 1 schematic ion trap according to the present invention;

Figure 2 flow diagram of the method according to the present invention;

Figure 3 another embodiment of the method according to the present invention; Figure 4 another embodiment of the method according to the present invention;

Figure 5 example of the signals in the ion trap;

Figure 6 a frequency representation for the present invention in a first embodiment;

Figure 7 a frequency representation in another embodiment;

Figure 8 an excitation scheme for the present invention;

Figure 9A-9C a first detection scheme according to the present invention;

Figure 10A-10C a second detection scheme according to the present invention; and

Figure 11 an exemplified signal result from the detection scheme of Figure 9 or 10.

Detailed Description

Referring to Figure 1 showing an ion trap according to the present invention. The ion trap 10 comprises a first electrode 18 built as ring electrode, a second electrode 14 built as first cap electrode and a third electrode 16 built as second cap electrode wherein the first electrode 18, second electrode 14 and third electrode 16 define a trapping volume 20 in order to trap the ions. A first RF signal supply 24 is connected to electrode 18, generating a RF signal storage field between the first electrode 18 and electrodes 14 and 16 to store the ions inside the trapping volume 20. Further, signal supplies 22, 26 might be connected to the second electrode 14 and/or the third electrode 16 in order alter the storage field. Therein, all the signals supplied are commonly connected to a control unit 28 in order to control generation of the respective signals. Therein, the signal supplies might be implemented as DDS modules or integrated in one DDS module. IInn ccoonnvveennttiioonnaall FFT image current measurements, an excitation signal is supplied either as a monopole signal to either electrode 14, 16 or differentially between 14 and 16 prior to detection. Therein, the excitation signal might be provided by a second signal supply or, as exemplified in Figure 1 and described above. As a result of the excitation signal the ions trapped in the trapping volume 20 are excited to oscillate around their equilibrium position in the ion trap 10 if the frequency of the excitation signal matches the oscillation ion frequency ω ₀ of the respective ions. Therein, the secular frequency ω ₀ is dependent on the mass of the ions and thus mass selective excitation of oscillation of ions can be facilitated. Due to oscillation of the ions in the trapping volume 20 an image current is induced into the second electrode 14 and the third electrode 16. The ion trap 10 further comprises a detector 30 which might be connected either to the second electrode 14 or to the third electrode 16. Alternatively, as exemplified in Figure 1 the detector 30 is connected to the second electrode 14 as well as to the third electrode 16 in order to detect the differential signal of the image current induced in the second electrode 14 and third electrode 16. Thus, excitation of the ions can be detected upon detection of the image current signal with the respective frequency.

In the present invention, the excitation signal is provided as parametric excitation inducing a parametric oscillation of the ions. Thereby, the excitation signal has a frequency is two-times the secular frequency or oscillation frequency of the ions, leading to ω _excite = 2 * ω ₀ . This is depicted in Figure 5 showing the excitation signal and the signal of the image current induced in one or both of the cap electrodes of the ion trap 10. From Figure 5 and also from the process of parametric oscillation it is known that the excitation signal ω _excite is at a different frequency than the excited oscillation of the ion and its resultant image current signal, thus enabling easy separation of the two signals and with minimal crosstalk or overlap which would degrade the detection sensitivity. Thus, it is not necessary anymore to time multiplex excitation and detection, meaning that excitation and detection can be performed simultaneously. In Figure 5 below the detected image current is shown in a full line for the initial response upon start of the parametric excitation of the ions. Thereby, due to the parametric excitation oscillation built up and upon continued excitation by the excitation signal a steady state develops shown in the dashed line.

Referring to Figure 2 showing the method for detecting ions in an ion trap 10, comprising: a) Ionizing ions in the ion trap SOI; b) Creating an RF storage field by applying a first RF signal to a first electrode of the ion trap S02; c) Applying an excitation signal to the ions in the ion trap S03; d) Detecting an image current signal induced to a second electrode, a third electrode or differentially between the second and third electrodes by oscillation of the ions excited by the excitation signal S04; wherein the excitation signal is a parametric excitation.

In step SOI, ionizing ions in the ion trap 10 is performed wherein every known scheme of ionizing ions can be utilized to provide ions to be trapped in the ion trap.

In step S02, a RF storage field is applied by applying a first RF signal to the first electrode 18 of the ion trap in order to trap the ions in the trapping volume 20.

In step S03, an excitation signal is applied to the ions in the ion trap wherein, as discussed above, the excitation signal is a parametric excitation with a frequency ω _excite = 2 * ω ₀ with ω ₀ being the oscillation frequency of the ions. S03 can also include a dipolar stimulus prior to parametric excitation.

In step S04, an image current signal is detected being induced into the second electrode 14 or the third electrode 16 or by detecting the differential between the second electrode 14 and the third electrode 16. From the detection signal, oscillation of the respective ion in the species or sample can be reliably detected. If ions with a secular frequency ω ₀ are present in the trapping volume 20 these ions will be excited by the excitation signal with ω _excite = 2 * ω ₀ , thereby inducing an image current signal into one or both of the second and third electrodes being detected by the detector 30. If the ion response signal is detected by detection of the image current signal it can be confirmed that ions with the respective oscillation frequency or secular frequency ω ₀ are present in the trapping volume 20. Therein, the oscillation frequency of the ions ω ₀ depends on the mass of the ions and thus a mass selective detection scheme is provided. Therein, according to arrow 32 in Figure 2, the steps S03 and S04 can be repeated one or several times using the same or different excitation frequencies in order to sweep over a frequency range of interest, detecting the ions in this range. Therein, sweeping can be performed continuously, stepwise or in a predetermined manner, or in a dynamically determined fashion wherein the excitation frequency and amplitude are adjusted during the course of a single measurement in direct response to the measured image current response of relevant ion masses.

Referring to Figure 3 showing a different embodiment of the method wherein the steps SOI to S04 are equal to the method as described with respect to Figure 2. In addition, after performing S04 an additional step S031 is performed. In step S031 ions excited by the excitation signal are expelled from the trapping volume 20 and further adjusting the frequency of the excitation signal is performed in order to provide a second excitation signal. Subsequently, in step S03 the second excitation signal is applied to the remaining ions in the trapping volume 20 in order to excite different ions in the species responsive to the second excitation signal, if present. Subsequently, in step S04 the induced image current signal is detected on the basis of the second excitation signal, wherein the steps S03, S04 and S031 can be repeated one or more times in order to cover a range of different excitation signal frequencies and detect respective ion species.

Referring to Figure 8 showing the excitation signal for different excitation frequencies sweeping over a respective frequency range addressing different ions having different masses thereby creating different image currents signals. Therein, the excitation signal is a parametric excitation having a two-times higher frequency than the image current signal as shown in Figure 8. Therein, upon start of the respective excitation signal, oscillation of the ions in the trapping volume 20 built up until the excitation signal is stopped either according to pre-set times, or by the signal reaching a set threshold level, or by loss of signal due to ejection of the ions from the storage volume.

Instead of sweeping across a frequency range either in a continuous manner or a stepwise manner as shown in Figure 8, it is also possible to address different ions having different masses by superposition of different excitation signals thereby simultaneously exciting oscillation of different ions in the ion species.

Referring to Figure 6 showing a frequency range relevant for the present invention. Therein, curve 34 showing the respective detection profile of the used detector, ω _max indicates the maximum frequency which is detectable by the detector, wherein ω _low indicates the upper cut-off frequency of the respective detector 30. Thus, due to the parametric excitation frequencies of the image current signal between ω _low /2 and ω _max can be detected leading to excitation frequencies between ω _low and 2 * ω _max . Increasing ω _max would lead to frequencies not being detectable anymore by the detector 30, wherein decreasing ω _low would lead to an unwanted overlap of the excitation signal with the detection range of the detector 30.

Referring to Figure 7, instead of considering the detection profile 34 itself, a notch filter 38 or a lowpass filter 40 can be used in order to shape the respective detection profile 34 accordingly and separate the image current signal from the excitation signal. Therein, upon use of a notch filter, the detection profile is transformed to curve 36a. Upon an excitation at ω _notch no detection of the excitation signal occurs due to the notch filter, wherein at the position of ω _notch /2 the image current signal can be reliably detected. Thus, by a notch filter 38 lower frequencies for the parametric excitation can be used even within the detection profile 34 of the detector 30. The same applies to the use of the lowpass filter 40 shaping the detection profile 34 to curve 36b depicted in Figure 7, separating the excitation signal from the image current signal even for lower frequencies.

Referring now to Figures 9A to 9C, in Figure 9C the ion response signal 100 is shown, showing the full response of the ions in the frequency domain for an ion species. If the frequency of the excitation signal is now slowly swept across the full frequency range of the ion response, or stepped in discrete steps smaller than the width of the ion resonance 100 as shown in Figures 9A and 9B, and the image response at 108 is detected over sufficient averaging period during each discrete frequency step or over a short period of the scan, then a derivative spectrum can be produced. Thus, referring to Figure 11 upon ion species with different ions having different masses close together, the differential signal 106 obtained by the detection scheme described with respect to Figure 9 leads to different curves with respect of the included ions in the species. If only one species of ions is present in the detection range swept by the excitation signal 102, a single peek 100 is shown. If two or more different ions having slightly different masses are present in the detection range, the ion response 100' is the sum of the individual ion response signals 101'. In the differential measurement signal 106' a clear second peek can be identified providing a clear indication for a mixture of ions also called an isobaric species including different ions having slightly different masses.

Referring to Figures 10A to 10C in another detection scheme the frequency of the excitation signal is switched between two different frequencies within the detection range. The excitation signal 102 is switched between an on- resonance and off-resonance with respect to the ion response signal 100. The switching frequency may be at a low frequency which is well below the amplifier passband. Thus, a composite signal is formed between on-resonance and off- resonance image current signals 110 which is synchronously demodulated at against the frequency switching signal to provide a measurement of total peak height. Thus, a novel method for detection of ions in an ion trap is provided using parametric excitation separating the excitation signal from the detection signal. Thereby, no time multiplexing is necessary anymore and simultaneous excitation and measurement is possible. Further than that, new detection schemes can be utilized in order to provide insights into the ion species.

Reference List:

10 ion trap

14 second electrode

16 third electrode

18 first electrode

20 trapping volume

22, 26 signal supplies

24 first RF signal supply

28 control unit

30 detector

32 arrow

34 detection profile 36a, 36b curve

38 notch filter

40 lowpass filter

100, 100' ion response signal

101' ion response signals

102 excitation signal

106 differential signal

106' differential measurement signal

108 image response

110 image current signals